Photoelectric sensors, sometimes referred to as scanners, are widely used in the fields of electrical and electronic controls in a wide variety of industrial applications. Some sensors are designed as two-part systems, wherein the light transmitting unit and light receiving unit are in separate housings which may be placed remote from each other to define an optical path for sensing the presence or absence of objects. In single-unit sensors, the light transmitter and receiver are combined in a single housing intended to be used with a retro-reflector or a reflective object that is positioned apart from the scanner to define the optical sensing path. The present invention is applicable either to two-part or single-unit scanners; and the preferred form, for purposes of economy, is a single-unit scanner. It is advantageous to provide a scanner construction that is low in cost, flexible in mounting techniques for use in a variety of applications, and rugged enough to withstand environmental factors to which it may be subjected in use. To these ends, a common type of sensor package has been provided in the prior art, known as a threaded barrel sensor. This type of package consists of a cylinder, typically 8, 12, 18 or 30 millimeters in diameter, and threaded over its entire length. Optical output/input is provided at one end via a suitable lens or lenses, and typically an electrical connection in the form of a cable is provided at the other end. Often an indicator LED is provided at the cable end of the barrel to show the ON/OFF status of the sensor output. The threaded length provides a convenient mounting mechanism using a pair of jam nuts to secure it in a simple mounting bracket.
Photoelectric sensors generally have one or two lenses, or a double lens, to collimate and focus the emitted and received light. Typically in prior art barrel sensors, the lenses are made of clear plastic or of glass, and are fastened inside the barrel housing using an intermediate lens holder mechanism, usually a molded plastic part.
Also inside the barrel there is usually a printed circuit board which contains the sensor electronics and the optoelectronic elements, which usually consist of an LED emitter and a phototransistor or photodiode receiver.
In such prior art barrel sensors, the usual manufacturing technique is to assemble the printed circuit board, install the lenses in the intermediate lens holder, install the lens holder in the barrel (often using an "O"-ring for sealing), install the indicator LED and cable in the rear end cap, install the printed circuit board in the barrel, install the rear end cap on the barrel, then encapsulate the entire assembly with epoxy. The epoxy encapsulation is used for environmental sealing primarily against moisture, but also against other contaminants or gases to which the unit might be exposed in use. In such prior art systems, the encapsulation step requires that all parts fit together and seal perfectly, or else the epoxy will leak into the optical chamber, and may also leak out of the barrel housing. It is therefore necessary to provide leakproof joints around the lenses, the lens holder and the optoelectronic elements to keep epoxy from leaking into them, and around the rear end cap, the indicator LED, and the cable entrance, to prevent epoxy from leaking out. A high degree of dimensional precision in the various parts is required to achieve this sealing. This is an expensive and time-consuming process, but a necessary one in the prior art.
In prior art barrel sensors, even the encapsulation process does not guarantee integrity against environmental influences, because such sensors can be vulnerable to moisture despite the encapsulation. Liquids can enter at epoxy-to-plastic boundaries by capillary action. A common entrance point for moisture is around the indicator LED. Constant thermal cycling of the sensor can cause slight gaps to open. Moisture is then drawn along the boundary over an extended period of time, which could take months, until it reaches a part of the circuit where it lowers the impedance and renders the sensor inoperative or faulty. This capillary action can also take place at each end of the barrel, at the lens boundaries, and at the cable entrance.